Mass spectrometers are used extensively in the scientific community to measure and analyze the chemical compositions of substances. In general, a mass spectrometer is made up of a source of ions that are used to ionize neutral atoms or molecules from a solid, liquid, or gaseous substance, a mass analyzer that separates the ions in space or time according to their mass or their mass-per-charge ratio, and a detector. Several variations of mass spectrometers are available, such as magnetic sector mass spectrometers, quadrupole mass spectrometers, and time-of-flight ion mass spectrometers.
Time-of-flight ion mass spectrometers (TOF-IMS) can detect ions over a wide mass range simultaneously. Mass spectra are derived by measuring the times for individual ions to traverse a known distance through an electrostatic field free region. In general, the mass of an ion is derived in TOF-IMS by measurement or knowledge of the energy, E, of an ion, measurement of the time, t1, that an ion passes a fixed point in space, P1, and measurement of the later time, t2, that the ion passes a second point, P2, in space located at a predetermined distance, d, from P1. Using an ion beam of known energy-per-charge E/q, the time-of-flight (tTOF) of the ion is tTOF=t2−t1, and by the ion speed. In a gated TOF-IMS uncertainty in tTOF may result, for example, from ambiguity in the exact time that an ion entered the spectrometer.
One method of attempting overcome this limitation in TOF-IMS utilizes a thin foil located at the entrance to spectrometer. The thin foil method works best with ions having sufficient energy to traverse the foil. Secondary electrons generated by the interaction of the ion with the foil are detected and provide an indication of when the ion entered the spectrometer. However, the foil method is not without its own limitations. These limitations include the requirement that the incident ion have sufficient energy to transit the foil, the energy degradation of the sample ion due to interaction with the foil, and the angular scattering of the sample ion due also to its interaction with the foil.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for resolving when the time that at least one ion entered the spectrometer without substantially reducing the energy of the at least one ion